Most rotary drilling methods require the use of drilling fluids. Drilling fluids perform several functions. The primary functions include cleaning the cuttings from the face of the drill bit, transporting the cuttings to the ground surface, cooling the drill bit, lubricating the drill bit and increasing the stability of the borehole.
Compressed air, foam, clear water and water-based mud are four general types of drilling fluids. Air and water generally satisfy the primary functions of a drilling fluid. However, additives must often be added to these fluids to overcome specific downhole problems. Air with additives is generally referred to as “foam.” A water-based drilling fluid with additives is commonly referred to as “drilling mud.” Another type of drilling fluid is the oil-in-water or oil-based mud.
Aqueous clay-based drilling fluids or muds are well-known in the prior art, as are different additives. Such fluids are comprised basically of water; a clay such as bentonite or sepiolite; lignosulfonate; a base such as NaOH; a densifier such as barite, or barium sulfate and possibly a salt containing a cation such as sodium or potassium. Other aqueous ionic compounds such as NaCl may also be present. These fluids are suitable for drilling at a pH level from about 8 to 11.5.
Water-based muds, plus appropriate additives, more or less fulfill the primary functions of drilling fluids, as described above. However, the primary disadvantage of using drilling mud are: a large volume of drilling fluid (water) is required, and a high potential for hole erosion exists. Also, the flush or return velocity of the drilling fluid, coupled with its viscosity, is potentially hazardous to erodible materials in boreholes.
The most common additive to form a water-based mud is bentonite. Bentonite consists of finely ground sodium bentonite clay. When mixed with water, the resulting slurry has a viscosity greater than water, possesses the ability to suspend relatively coarse and heavy particles, and tends to form a thin, very low permeability cake on the walls of the borehole. Bentonite is generally available in a standard grade and a high yield grade, which contains organic polymers and generally produces the same viscosity as the standard grade, with about one-half the amount of bentonite. However, the standard grade bentonite may contain peptizing agents and organic additives that may be environmentally unacceptable in some applications.
Drilling muds have four basic properties that determine the behavior of the mud as a drilling fluid: viscosity, density, gel strength, and filtration. Simple field tests can be employed to test for viscosity and density. As a general rule, viscosity should be maintained as low as possible to provide the required hole stability and fluid loss control. Thin mud generally does the best job of cleaning the drill bit, but thick muds are needed to remove coarse gravel from the hole.
Density is defined as the weight per unit volume of drilling fluid. An increase in the density of the drilling mud is a measure of how much drilled material is being carried in suspension and re-circulated. Excess suspended solids are objectionable for several reasons. First, the cuttings are generally abrasive and increase wear on the mud pump, drill string and drill bit. Re-grinding of cuttings also tends to decrease the rate of drilling progress. A thicker filter cake will be formed on the walls of the borehole as a result of the higher concentration of solids. As a result of the higher hydrostatic pressure caused by the higher concentration of solids, hydraulic fracturing of the formation is likely to occur. Also, a denser fluid has greater buoyancy, thereby making it less likely that the cuttings will settle out in the mud pit.
Gel strength is the measure of the capability of a drilling fluid to hold particles in suspension after flow ceases. Gel strength results from the electrical charges on the individual clay platelets. In a bentonite mud in which the particles are completely dispersed, essentially all the bonds between particles are broken while the mud is flowing. When the mud pump is shut off and flow ceases, the attraction between clay particles causes the platelets to bond to each other. This coming together and bonding is termed flocculation. This edge to face flocculation results in an open card-house structure capable of suspending cuttings and sand and gravel particles. This property also suspends finely ground, high specific gravity material when high density drilling muds are required. The capability of keeping cuttings in suspension prevents sandlocking the tools in the borehole while drill rods are added to the string and minimizes sediment collecting in the bottom of the hole after reaming. A drawback to this property is that cuttings do not readily settle out of the drilling mud and may be re-circulated, thus resulting in grinding of particles by the drill bit, increased mud density, increased mud pump wear, and lower penetration rate.
Filtration refers to the ability of the drilling fluid to limit fluid loss to the formation by deposition of mud solids on the walls of the hole. During drilling operations, the drilling fluid tends to move from the borehole into the formation as a result of hydrostatic pressure which is greater in the hole than in the formation. As the flow of drilling fluid occurs, the drilling fluid solids are deposited on the walls of the borehole and thereby significantly reduce additional fluid loss. The solids deposit is referred to as a filter cake. The ideal filter cake is thin with minimum intrusion into the formation. The thickness of the filter cake for a particular mud is generally a function of the permeability of the formation. For example, the filter cake in a clay interval of the borehole would be thinner than in a sand interval. A thick filter cake has a number of disadvantages which include the possibility that the cake may be eroded by circulating drilling fluid, may cause the drill pipe to stick, or may cause reduced hydrostatic pressure and partial collapse of the walls of the borehole during tool removal. Also, the re-entry of drilling equipment into the borehole lined with a thick filter cake could result in a pressure surge with an accompanying increased potential for hydrofracture of the formation.
Water quality, method of mixing and mud pit design are important to the effective use of water-based drilling muds. Effective dispersion of and hydration of the drilling mud solids is dependent on proper mixing. Sprinkling or pouring the dry additives into the water and relying on the drill rig pump to mix will result in a lumpy mud with excessive additives for the mud properties achieved.
It is highly desirable to provide a method for drilling a borehole, including the step of providing an inexpensive additive for water-based drilling fluid or drilling mud that reduces the viscosity of drilling fluid or drilling mud, increases the stability of the borehole and/or reduces fluid loss.